1. Field of the Invention
The present invention relates to a developing apparatus for use in an electrophotographic image forming machine and to an image forming machine incorporating the developing apparatus. More particularly, the present invention relates to a trickle developing apparatus that gradually supplies fresh developer and gradually discharge deteriorated developer and to an image forming machine incorporating the developing apparatus.
2. Description of the Related Art
As developing systems employed for electrophotographic image forming machines, the one-component developing system in which toner is used as the main component of the developer and the two-component developing system in which toner and carrier are used as the main components of the developer are known.
The two-component developing system that uses toner and carrier, in which the toner and carrier are charged by friction contact therebetween to predetermined polarities, has a characteristic that the stress on the toner is less than that in the one-component developing system that uses a one-component developer. Since the surface area of the carrier is larger than that of the toner, the carrier is less contaminated with the toner attached to the surface thereof. However, with the use for a long period, contamination (spent) attached to the surface of the carrier increases, whereby the capability of charging the toner is reduced gradually. As a result, problems of photographic fog and toner scattering occur. Although it is conceivable that the amount of the carrier stored in a two-component developing apparatus is increased to extend the life of the developing apparatus, this is undesirable because the developing apparatus becomes larger in size.
To solve the problems encountered in the two-component developer, Patent document 1 discloses a trickle developing apparatus being characterized in that fresh developer is gradually replenished into the developing apparatus and developer deteriorated in charging capability is gradually discharged from the developing apparatus, whereby the increase of the deteriorated carrier is suppressed. The developing apparatus is configured to maintain the volume level of the developer inside the developing apparatus approximately constant by discharging an excessive amount of deteriorated developer using the change in the volume of the developer. In the trickle developing apparatus, the deteriorated carrier inside the developing apparatus is gradually replaced with fresh carrier, and the charging performance of the carrier inside the developing apparatus can be maintained approximately constant.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. Sho 59-100471
However, when an image having a high coverage ratio, such as a photographic image, is printed in the above-mentioned trickle developing apparatus, toner inside the developing apparatus is consumed abundantly to form an image having a high coverage ratio. Although toner is replenished into the developing apparatus by replenishing fresh developer containing toner and carrier to compensate for the toner consumed abundantly, the carrier replenished simultaneously with the toner is not consumed but remains inside the developing apparatus.
If images having high coverage ratios are printed out continuously, the accumulated amount of carrier inside the developing apparatus increases, the volume level of the developer inside the developing apparatus rises gradually. When the volume level reaches the discharge level of a trickle discharging mechanism, the developer is discharged using the trickle discharging mechanism. If the discharging amount is set higher so as to print out images having high coverage ratios, when images having ordinary coverage ratios are printed out, the discharging amount using the trickle discharging mechanism increases and the developer inside the developing apparatus is used wastefully. For this reason, the discharging amount using the trickle discharging mechanism is usually set lower so as to print out images having low coverage ratios. Accordingly, if images having high coverage ratios are printed out continuously, when the amount discharged using the trickle discharging mechanism is small, developer discharge is not carried out sufficiently.
The developer not discharged sufficiently using the trickle discharging mechanism is accumulated inside the developing apparatus beyond the discharge level of the trickle discharging mechanism. If the level of the developer exceeds an overflow level, that is, a regulating gap between the sleeve of a developing roller and a regulating plate opposed to the sleeve, the developer overflows over the regulating gap, thereby damaging the image forming machine.
It is technically possible to use detecting means, such as a level sensor (for example, a pressure sensor), to detect the volume level of the developer existing inside the developer tank. However, in that case, there occurs a problem that the configuration becomes complicated or the cost becomes high.
Accordingly, in the case that images having high coverage ratios are printed out continuously in a trickle developing apparatus that uses a two-component developer, the technical problem to be solved by the present invention is to provide a developing apparatus and an image forming machine capable of predicting how much carrier is accumulated inside the developer tank beyond the discharge level of the trickle discharging mechanism.